1. Field of the Invention
The present invention relates to an electrostatic holding apparatus, and more particularly to an electrostatic holding apparatus useful for a process performed during fabrication of semiconductor devices, liquid-crystal devices, etc., which apparatus can strongly attract and hold a conductive, semiconductive, or insulating object by means of electrostatic force at any temperature ranging from a low temperature to a high temperature and which enables easy attachment/detachment of the object.
2. Description of the Related Art
Recently, processes for fabricating semiconductor devices, liquid-crystal devices, etc.; especially processes of drying etching, ion implantation and vapor deposition, have been more and more automated and performed as dry processes. Under such circumstances, the number of fabrication processes performed under vacuum has increased.
Meanwhile, positional accuracy at the time of patterning has become more important since the diameters of silicon wafers and glass plates serving as substrates have increased and the degree of integration of circuits and the degree of fineness of patterns have increased. Therefore, vacuum chucks have been used to transport wafers or to attract and fix wafers. However, vacuum chucks have the following drawbacks. Vacuum chucks cannot be used under a vacuum environment because of impossibility of creating a pressure difference. Although vacuum chucks can be used under a non-vacuum environment, a sucked wafer undergoes a local distortion because the wafer is sucked locally, with the result that accurate positioning becomes difficult. Therefore, vacuum chucks are not suitable for recently developed processes for fabricating semiconductor devices and liquid-crystal devices.
Recently, as a device that has overcome the above-described drawbacks, there has been widely noticed and put into practical use an electrostatic holding apparatus which transports and/or attracts and fixes a wafer by means of electrostatic force. In a recent process for fabricating semiconductor devices and liquid-crystal devices, with an increase in the degree of fineness of devices, the flatness of wafers and glass plates serving as substrates has become more and more important. Therefore, employment of electrostatic holding apparatuses has been considered in order to perform a straightening correction for improving the flatness of wafers and glass plates.
Such a straightening correction for improving the flatness of wafers and glass plates requires a very strong electrostatic force. In order to meet such a requirement, there has been proposed a technique in which titania (TiO.sub.2) is mixed into an insulating material such as alumina in order to decrease the volume resistivity to thereby increase the electrostatic attraction force (see Japanese Patent Application Laid-Open (kokai) Nos. 62-94953, 2-206147, 3-147843, and 3-204924).
As described above, when alumina containing titania is used for an insulating dielectric layer of an electrostatic attraction portion, the volume resistivity of the insulating dielectric layer decreases and a weak current flows therethrough, so that an increased electrostatic force is generated due to the Johnson-Rahbek effect. However, since titania is a semiconducting substance, movement velocity of charges decreases, so that even when the volume resistivity is optimized, the response characteristics (time required for reaching saturated attraction force, time required for annihilation of residual attraction force) at the time of stopping application of voltage deteriorate. Consequently, an article becomes difficult to remove from the attraction surface of the electrostatic holding apparatus. This deterioration in the response characteristics becomes remarkable when the electrostatic holding apparatus is used at low temperature.
Decreasing the volume resistivity of the insulating dielectric layer to, for example, 1.times.10.sup.8 .OMEGA..multidot.cm requires addition of about 25 wt. % TiO.sub.2 ; however, TiO.sub.2 must not be incorporated, as an impurity, into semiconductor devices during a process of fabricating the semiconductor devices. When the temperature of a semiconductor wafer to be held is equal to or higher than room temperature the volume resistivity of the wafer is excessively low, so that a strong leakage current flows from a holding apparatus to the wafers, resulting in breakage of circuits formed on the wafer.
Further, due to a difference in coefficient of thermal expansion between titania and alumina, cracks and/or pores are generated in sintered alumina, so that the withstand voltage is low.